1. Field of the Invention
The present invention relates to a digital information input method especially convenient for a camera which is controlled by means of digital information.
2. Description of the Prior Art
Generally, a photographic instrument such as camera is so designed that, for controlling exposure, the information concerning the brightness of the object to be photographed or the speed of the film to be used as well as the set informations concerning the exposure time or the aperture value are put in the exposure operation means so as to control the aperture value or the exposure time. It is typical that such data for the exposure control are once converted into APEX value and operated for the sake of the simplification of operation. The APEX values are the exposure time, the aperture value, the film speed and the object brightness replaced by the arithmetic series system for the sake of the simplification of operation as is shown in FIG. 1. The exposure time is given in seconds, the aperture value given in F.No., the film speed given in ASA sensitivity and the object brightness given in foot lamberts in multiple series system. In the drawing, the values corresponding to the APEX values are represented by Tv for the exposure time, Av for the aperture value, Sv for the film speed and Bv for the object brightness.
In accordance with the APEX value series system, it is sufficient to carry out the exposure operation only by addition and subtraction so as to satisfy the following relation so that no matter whether it is carried out electrically or mechanically the exposure operation becomes very simple. EQU Bv + Sv = Tv + Av (1)
Even for the recently proposed digital approach to exposure operation the APEX value series system is applicable with remarkable efficiency in view of simplifying the operation device. Although different from the analog system, the theoretical accuracy of the digital system is naturally limited due to the bit capacity of the digital system. In consequence, when it is desired to obtain a certain desired accuracy in the digital system it is sufficient to increase the bits for representing figures below "1" in APEX value, whereby an extremely high accuracy is not necessary if the device error, the measurement error and so on are taken into consideration.
When 7 bits are used for converting the factors for exposure control into APEX values as is shown in FIG. 2, an accuracy up to "1/8" in APEX value can be otained, by giving the weights "1", "2", "4" and "8" respectively to the four bits of the higher order a.sub.1, a.sub.2, a.sub.4 and a.sub.8 and the weights "1/2", "1/4" and "1/8" respectively to the three bits of the lower order a1/2, a1/4 and a1/8.
Thus by converting the measured light information, the film speed information, the exposure time information and the aperture value into digital amount of 7 bits as is shown in FIG. 2 so as to carry out the operation for the exposure control, the exposure operation can be carried out with relatively high accuracy.
Thus the value below "1" in APEX value the values "1/8", "2/8", "3/8", "4/8", "5/8", "6/8" and "7/8" can be treated with the three bits of the lower order a1/2, a1/4 and a1/8. Further, it is the present market tendency to set the sensitivity of the film used for the photography in the values by 1/3 of a step in the APEX value. Thus it is necessary to set the film sensitivity by 1/3 step precision in APEX value as ASA 16, 20, 25, 32, 40, 50, 64, 80, 100, 125, 160 and 200.
However, if the factor relative to the film speed is set with 1/3 step precision while other factors for the exposure control are set with 1/8 step precision, the matching between the factors for the digital operation can not be obtained. Therefore, it becomes necessary to adopt a more complicated operation including multiplication and division. If on the other hand, the result of the operation for the actual control is obtained with 1/8 step precision, such complicated operation as mentioned above becomes unnecessary.
A purpose of the present invention is to eliminate the above mentioned shortcoming by converting the data with 1/3 step precision into the data with 1/8 step precision in an approximate way.